Coin operated telephone stations are interfaced with a central office by a two wire loop circuit When the handset is lifted off the hook, the central office supplies a very small DC loop current, typically 23 mA minimum, which is available to the coin operated telephone for use as a source of power. This source of power, however, is interrupted during a number of events in the course of a typical payphone call.
It is important for a local station such as a payphone to be able to operate in spite of such power interruptions or open switch intervals ("OSI"). Where alternate sources of power are not available, it is critical to be able to operate from the line power even though that power will be interrupted several times during each call.
In the past, line powered phones have used large capacitors to provide a supply of stored power which is used to power control electronics during open switch intervals. In one such phone, an approximately 100,000 microfarad (uF) storage capacitor is employed.
The open switch intervals vary according to the control switching system in use at the central office. For older switching systems, open switch intervals are typically 350 milliseconds (ms) but under certain conditions can extend up to 600 ms. In fact, under worst case conditions, such intervals have been measured to last as long as 4 seconds.
To address these OSI conditions, large capacitors, sometimes called super caps have been employed. Usage of large capacitors, however, may result in a number of unsatisfactory effects. For example, in order to initially charge large capacitors from the phone lines, many seconds are required. Consequently, if the large capacitor is not sufficiently charged when the payphone customer picks up the receiver, an initial dial tone will be delayed until the capacitor is adequately charged. The delayed dial tone is a source of confusion to the customer and may result in lost sales if the user assumes the phone is dead and hangs up because no dial tone is immediately heard. To try to avoid this problem, one phone employing a large capacitor trickle charges that capacitor while the phone is on hook. On hook, this phone draws about 1.6 milliamperes (mA) to charge its capacitor. While this arrangement allows the capacitor to be fully charged when the phone is taken off hook, thereby eliminating the delayed dial tone, it violates certain existing on hook current specifications which limit the on hook current to a maximum of approximately 1 microampere (uA). Besides not meeting these specification, an arrangement requiring a relatively large on hook current at some point becomes impractical, as the current available from the central station which provides the line power is limited.